Liquid crystal compositions

ABSTRACT

Liquid crystal cells containing mixtures of cholesteric optically active p-alkoxybenzylidene-p&#39;&#39;-aminobenzonitrile compounds with nematic liquid crystal compounds form cholesteric liquid crystals which change to the nematic state upon application of an electric field. Electro-optic devices including such liquid crystals have low voltage requirements and rapid response times.

United States Patent 1 1 1111 3,923,685

Oh et al. Dec. 2, 1975 1 LIQUID CRYSTAL COMPOSITIONS 3,815,972 6/1974Hsieh 350/160 LC [75] Inventors: Chan S00 Oh, Fullerton, Calif.;

Edward F. Pasierb, Hamilton Square, Ni Primary Examiner-Harvey E.Behrend Assistant ExaminerRalph Palo [73] AsslgneeI RCA Comm-anon NewYork Attorney, Agent, or Firm-G. H. Bruestle; B. E. Morris [22] Filed:Oct. 18, 1973 [21] Appl. No.: 407,546

Related U.S. Application Data [57] ABSTRACT [62] Division of Ser. No.298,339, Oct. l7, I972. Pat. No.

Liquid crystal cells contalnlng m xtures of cholestenc 152 U.S. c1...252/408 LC; 252/299; 252/408 LC; Optically active P- Y Y -P 350/160 LCaminobenzonitrile compounds with nematic liquid 1511 Int. Cl. C09K 3/34Crystal Compounds form Cholesteric liquid Crystals [58] Field of Search252/408 LC, 299; which Change to the nematic State upon application of350/160 LC an electric field. Electro-optic devices including suchliquid crystals have low voltage requirements and [56] Referencescitedrapid response times.

UNITED STATES PATENTS 3,796,479 3/1974 Helfrich 350/150 3 Claims, 3Drawing Figures ISOTROPIC -mol CHOLESTERIC COMPOUND- LIQUID CRYSTALCOMPOSITIONS This is a division of application Ser. No. 298,339, filed-17-72, now US. Pat. No. 3,792,915.

This invention relates to electro-optic devices which respond toapplication of an electric field. More particularly, this inventionrelates to cholesteric liquid crystal compounds which, when incorporatedinto an electrooptic device, change to the nematic state in the presenceof an electric field.

BACKGROUND OF THE INVENTION Certain cholesteric liquid crystalcompositions are known which change to the nematic state uponapplication of an electric field. Wysocki et al. in Phys. Rev. Letters20 No. 19, page 1,024 (1968) reported that cholesteryl chloride, aloneor in admixture with other cholesteryl derivatives including cholesterylnanoate and cholesteryl oleyl carbonate, underwent a change from thecholesteric to the nematic state when an electric field was applied.Thus, an electro-optic cell including such materials changed from alight scattering, opaque appearance to a clear, colorless appearancewhen a direct current field of about 10 V/cm was applied. Such cellsthus require a fairly high voltage and they have the furtherdisadvantage that they have limited use temperature ranges. Thus, asimilar electro-optic device which would be operative over a wider usetemperature range, including room temperature, and would require lowervoltage to operate than those known heretofore, would be highlydesirable.

SUMMARY OF THE INVENTION We have discovered that mixtures of certainasymmetric, optically active, p-alkoxybenzylidene-paminobenzonitrilecompounds with nematic liquid crystals which have a strong positivedielectric anisotropy, particularly one or more alkoxy-oracyloxybenzylidene anils, form a cholesteric liquid crystal which, onapplication of an electric field, forms a nematic liquid crystal. Whenthe electric field is removed, the mixture reverts to the cholestericphase. The electric fieldinduced phase changes occur very rapidly andrequire only low fields to operate electro-optic devices incorporatingthem.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is a cross-sectional view of anelectro-optic device embodying the invention.

FIG. 2 is a phase diagram of the transition temperatures of mixturesdescribed hereinafter.

FIG. 3 is a diagram which shows the variation of voltage required toclear an electro-optic device with changing proportions of cholestericand nematic compounds.

DETAILED DESCRIPTION OF THE INVENTION The asymmetric optically activep-alkoxybenzylidene-p'-aminobenzonitrile compounds useful in the presentmixtures have the formula wherein n is an integer from 1 to 5. Theasymmetric carbon atom can have either the R or S configuration.Mixtures of these compounds can also be employed, which generally willreduce the temperature of the transition points.

The nematic liquid crystal compounds particularly useful in the presentmixtures are alkoxyand acyloxybenzylidene anils having the formula Thedesired alcohol is reacted with phosphorous tribromide to form thecorresponding bromide; the bromide is reacted with p-hydroxybenzaldehydein base to form the corresponding p-alkoxybenzaldehyde; and tinally thep-alkoxybenzaldehyde is reacted with paminobenzonitrile to form thedesired p-alkoxybenzylidene-p'-aminobenzonitrile. These reactions aresummarized by the following scheme II:

H c 5 l OH 9 C H ClH(CH One or more asymmetric, optically activep-alkoxybenzylidene-p-aminobenzonitrile compounds are then admixed withone or more alkoxyor acyloxybenzylidene anil compounds to form theliquid crystal mixturesuseful in the invention. The addition of one ormore nematic compounds to the cholesteric compound results in areduction of smectic thermal stability and an increase in themesomorphic temperature range. The mixture can contain from about 5 toabout 80%, preferably about 20 to about 60% by weight of the mixture ofthe optically active component, which in general gives the mostpractical use temperature range for electro-optic devices. Mixturescontaining from about 40 to about 50 percent by weight of the opticallyactive compound are particularly preferred.

An embodiment of an electro-optic device is shown with reference toFIG. 1. The liquid crystal cell 10 consists of front and backtransparent support plates 11 and 12 respectively. The support plates 11and 12, which may be of glass, quartz, sapphire, plastic or the like,are parallel and spaced apart by a distance of about one-fourth one-halfmil by means of a polytetrafluoroethylene spacer 13. On the innersurface 14 of the back plate 12 is a transparent conductive electrode15. On the inner surface 16 of front plate 11 is a transparentconductive electrode 17. The electrodes and 17 are the means by which anelectric field is applied to the device and are conventionally made of atransparent film of tin oxide, indium oxide and the like.

The space 18 between the plates 11 and 12 is filled with the liquidcrystal mixture described hereinabove, which filled space is then sealedby means of an epoxy cement.

In operation, the liquid crystal cell appears opaque in the absence ofan electric field. When a suitable electric field is applied thereto,the texture of the liquid crystal changes from a light scattering stateto a transparent state. The response time for these changes is afunction of the applied voltage, the temperature, and the concentrationof the optically active compound in the liquid crystal mixture. Ingeneral, application of from about 30 60 volts alternating current (60Hz) or direct current at room temperature results in a transition plasechange in 100 milliseconds or less. Surprisingly, the higher theconcentration of the optically active compound in the present liquidcrystal mixture, the lower the voltages required to clear the cell.

When the electric field is removed, the mixture returns to the lightscattering state. This time, referred to as relaxation time, also variessomewhat depending on the magnitude of the voltage applied, thetemperature, and the concentration of the optically active compound inthe liquid crystal mixture; in general, relaxation times vary from about0.1 to about 0.5 second at room temperature.

Both response time and relaxation time decrease with increasingtemperature. For example, a mixture which required about 300milliseconds for response at room temperature using an electric field of30 volts required only 40 milliseconds at 55C. and 12 milliseconds at73C. The same mixture had a relaxation time of 200 milliseconds at roomtemperature and only 40 milliseconds at 73C.

The electro-optic device as described hereinabove can be incorporatedinto various displays, such as electronic window shades, advertisingdisplays, numeric indicators and the like.

The invention will be further illustrated by the following examples butit is to be understood that the invention is not meant to be limited tothe details described therein. In the examples, parts and percentagesare by weight unless otherwise noted.

The transition temperatures of the compositions prepared in the exampleswere determined using a Thomas-Hoover melting point apparatus, adifferential scanning calorimeter and a polarizing hot stage microscopein conventional manner.

EXAMPLE 1 Part A Preparation of (S)-4-methylhexanol-l A Grignard reagentwas prepared by adding 302 parts of (S)-l-bromo-2-methylbutane over 1.5hours to 5 1 parts of magnesium turnings in 650 parts by volume ofdiethyl ether. The mixture was stirred at ambient temperatures for onehour and chilled to 30C. A cold solution of 110 parts of ethylene oxidein parts by volume of ether was added at a rate so as to maintain thetemperature below C. When the addition was complete, the mixture wasallowed to come to room temperature and stirred for three hours. Part ofthe ether was evaporated, 300 parts by volume of benzene were added andthe mixture was distilled until the temperature of distillation reached50C. An additional 200 parts by volume of benzene were added anddistilling was continued until the distillation temperature reached 65C.The mixture was cooled to room temperature, poured into 2,500 parts ofice water and 650 parts of dilute sulfuric acid (1:3) were added. An oillayer separated out, which was collected and dried over sodium sulfate.The aqueous layer was treated with sodium chloride to obtain furtheroily product which was also collected.

The combined crude product mixture was distilled under vacuum. A 46%yield (100 parts) of (S)-4- methylhexanol-l boiling at 75-85C/20 mm Hgwas obtained.

Part B Preparation of (S)-l-bromo-4-methylhexane (S)-4-Methylhexanol-l(90.5 parts) prepared as above was cooled to 20C. and 70.23 parts ofphosphorous tribromide added dropwise while maintaining the temperaturebelow 0C. The reaction mixture was stirred overnight at roomtemperature. The resultant orange-yellow solution was poured into 8,000parts of ice water and stirred. The oily layer which formed wascollected, washed with cold sulfuric acid, shaken with parts ofpotassium carbonate, filtered and distilled.

A 46% yield (62.6 parts) of (S)-l-bromo-4-methylhexane distilling at67C./20 mm Hg was obtained.

Part C Preparation of (S)-p-(4-methylhexoxy)benzaldehyde A solution of71 parts of p-hydroxybenzaldehyde in 500 parts by volume of warmcyclohexanone was prepared and 90 parts of (S)-l-bromo-4-methylhexaneand 250 parts of anhydrous potassium carbonate stirred in. The mixturewas refluxed while stirring vigorously for one-half hour. The resultantdark brown mixture was filtered and the solvent removed below 70C.

Distillation of the crude product was carried out at l20C./0.07 mm Hg.The distillate was dissolved in ether, successively washed with sodiumhydroxide and water, dried and redistilled.

A 53% yield (59 parts) of colorless (S)-p-(4-methylhexoxy)benzaldehydewas collected having a boiling point of l289C./0.08 mm Hg. The specificrotation [04],, was 7.40 and the density (1., was 0.9866. A single spotwas obtained on a thin layer chromatogram (silica gel/chloroform).

Part D Preparation of(S)-p-(4-methylhexoxy)benzylidene-p'-aminobenzonitrile(S)-Para-(4-methylhexoxy)benzaldehyde (55 parts), 29.5 parts ofp-aminobenzonitrile and 100 parts by volume of benzene were charged to aDean-Stark trap and refluxed for 2 hours. 0.4 Part of water wascollected in the trap. The resultant orange solution was distilled to 1remove benzene and the liquid remaining cooled to room temperature. Theproduct was a turbid yellow liquid which turned yellow-brown on warming.This product was dissolved in isopropanolzhexane and cooled in dry ice.The yellow crystals which separated were collected and recrystallizedtwice from cold isopropanol to give white crystals of(S)-p-(4-methylhexoxy)benzylidene-p-aminobenzonitrile.

The compound prepared as above was found to have three temperatures oftransition; crystal to smectic at 40C., smectic to cholesteric at 48C.and cholesteric to isotropic at 65.5C.

EXAMPLE 2 The procedure in Example 1 was followed except repeating theprocedure of Parts A and B to obtain (S) lbromo-6-methyloctane, whichwas then reacted following the procedure of Parts C and D.

The product, (S)-p-(6-methyloctoxy)benzylidene-paminobenzonitrile, hadtwo transition temperatures; crystal to smectic at 34C. and smectic toisotropic at 77C.

EXAMPLE 3 Preparation of (S)-3-methylpentanol-1 A Grignard reagent wasprepared by reacting 26.7 parts of magnesium turnings and parts of(S)-lbromo-2-methylbutane in 300 parts by volume of diethyl ether. Afterrefluxing for two hours, 50 parts of paraformaldehyde was evaporatedinto the above reaction mixture over several hours. The resultant slurrywas poured into 1,000 parts of ice water. 650 Parts of dilute sulfuricacid were then added. The oily product was collected. The aqueous layerwas treated with sodium chloride to separate additional oil which wastaken up in diethyl ether. The ether layer was evaporated to remove thesolvent and the remaining oil added to the crude product. The combinedproduct mixture was treated with potassium carbonate, filtered anddistilled to remove the solvent and finally distilled under vacuum togive (S)-3-methylpentanol-l as an oil.

Part B The corresponding(S)-p(3-methylpentoxy)benzylidene-p-aminobenzonitrile was preparedfollowing the procedure of Parts B, C and D of Example 1. This compoundhad a melting point of 63C. By melting and supercooling, a cholesterictransition temperature of 50C. was noted.

EXAMPLE 4 (S)-Para-(5-methylheptoxy)benzylidene-p'- aminobenzonitrilewas prepared from (S)-3-methylpentanol-l following the procedure ofExample 1.

This compound had three transition temperatures; crystal to smectic at55C., smectic to cholesteric at 59C. and cholesteric to isotropic at665C.

EXAMPLE 5 (S)-Para-(2-methylbutoxy)benzylidene-p'- aminobenzonitrile wasprepared from (S)-l-bromo-2- methylbutane following the procedure ofExample 1.

This compound had a melting point of 57C. By melting and supercooling, acholesteric transition temperature of 39C. was noted.

EXAMPLE 6 To a nematic mixture of equimolar amounts ofpethoxybenzylidene-p-aminobenzonitrile,p-n-butoxybenzylidene-p'-aminobenzonitrile andp-noctanoyloxybenzylidene-p'-aminobenzonitrile was added(S)-p-(4-methylhexoxy)benzylidene-p'- aminobenzonitrile in variousamounts and the transition temperatures measured.

The data is summarized in FIG. 2 which is a phase diagram of themixtures wherein the ordinate varies with the temperature and theabscissa varies with the mol percent of the optically active cholestericcompound. The smectic mesomorphism gives way to cholesteric mesomorphismexclusively as increasing amounts of the nematic mixture are present.The asterisks denote calculated values for transition temperatures basedon the standard cryoscopic equation wherein x is the mol fraction ofsolvent (in this case the nematic mixture), L; is the heat of fusion andR is 1.9869 cal/mol. Accordingly, a 50 mol percent mixture of thecholesteric compound in the nematic mixture will have the lowest crystalto cholesteric transition temperature.

EXAMPLE 7 Liquid crystal cells were constructed of a /2 mil thick layerof the mixtures of Example 6 between two transparent tin oxide coatedglass plates which were connected to a source of alternating currentoperating at 60 Hz.

FIG. 3 is a diagram which shows the voltage required to clear the cellfor various mixtures. An incident light beam was directed normal to thecell through a polarizer and a crossed analyzer held between aphotomultiplier detector. When the cell is extinct with the crossedpolarizer, the cell has been cleared. In FIG. 3 the ordinate varies withrelative brightness, expressed in arbitrary units, and the abscissavaries with the voltage required to clear the cell. In the diagram,Curve A represents a mixture containing mol percent of the cholestericcompound; Curve B represents a mixture containing 40 mol percent of thecholesteric compound; Curve C represents a mixture containing 60 molpercent of the cholesteric compound and Curve D represents a mixturecontaining 80 mol percent of the cholesteric compound.

According to the data there is an increase in brightness with increasingconcentration of the cholesteric compound in the mixture in the absenceof an electric field. An increasing voltage is applied, there is a sharpdrop in brightness for all the mixtures. However, less voltage isrequired to clear the cell when the concentration of the cholestericcompound in the mixture is increased.

EXAMPLE 8 A cholesteric mixture is prepared from equimolar amounts ofthe products of Examples 1 and 5, admixed with the nematic mixture as inExample 6. A liquid crystal cell including this mixture turns fromopaque to clear when an electric field of 50 volts is applied, whichreverts to the light scattering state when the electric field isremoved.

We claim:

1. A cholesteric mixture comprising from 5-80 percent by weight of anoptically active compound of the formula octanoyloxybenzylidene-p'-aminobenzonitrile.

1. A CHOLESTERIC MIXTURE COMPRISING FROM 5-80 PERCENT BY WEIGHT OF ANOPTICALLY ACTIVE COMPOUND OF THE FORMULA1-(CH3-CH2-CH(-CH3)-(CH2)N-O-),4-((4-(NC-)PHENYL)-N=CH-)BENZENE WHEREINN IS AN INTEGER FROM 1 TO 5 AND A NEMATIC LIQUID CRYSTAL HAVING POSITIVEDIELECTRIC ANISOTROPY COMPOSITION.
 2. A mixture according to cLaim 1wherein the nematic liquid crystal composition is selected from thegroup consisting of alkoxy- and acyloxybenzylidene anils.
 3. A mixtureaccording to claim 1 wherein the optically active compound is(S)-p-(4-methylhexoxy)benzylidene-p''-aminobenzonitrile and the nematicliquid crystal composition comprises an equimolar mixture ofp-ethoxybenzylidene-p''-aminobenzonitrile,p-n-butoxybenzylidene-p''-aminobenzonitrile andp-n-octanoyloxybenzylidene-p''-aminobenzonitrile.